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8/13/2019 Paper - Effect of Uit on Fatigue Life in Web-gusset Welded Joints
Fig. 2 Configuration of web-gusset welded joints. (unit: mm)
2.2 Fatigue test
Fatigue tests were carried out based on the 14S - N testing method as regulated in JSME
S 002-1994, the standard method of statistical fatigue testing(8)
. Fatigue tests were
conducted at room temperature under a load frequency f of 4 to 9 Hz and a stress ration R of
0.1 using a servo-hydraulic fatigue machine with a capacity of 1500 kN. Fatigue tests were
terminated in cases where the number of load cycles reached 10 million. During fatigue
tests, the strain amplitude of each weld toe was measured with a strain gage attached at a point 5 mm from the weld toe in order to detect a fatigue crack initiated at the weld toe. The
crack initiation life at a weld toe was defined as the number of cycles in which the value R s,
calculated by using Eq. (1), decreased 5%(9)
.
1000
×=ε
ε iS R (1)
where ε i is the strain amplitude during the fatigue test and ε 0 is the value of the initial strain
amplitude.
3. Results and Discussion
3.1 Fatigue test results
Figure 3 shows an S - N curve diagram for the UIT specimens. For comparison, the
results for the as-welded (AW) specimens are also shown in this figure. The fatigue lives of
web gusset welded joints were greatly improved throughout the whole stress range by the
UIT. The fatigue lives of the UIT specimens were increased more than tenfold in the stress
range where ∆σ is equal to 175 MPa or less. Also, the fatigue limit of web gusset welded
joints increased from 53 MPa to 111 MPa.
Fig. 3 Effect of UIT on fatigue life.
1 2
700
1 7 2
110
1 0 0
1 2
Loading direction
40
50
60
708090
100
200
300
10
5
10
6
10
7
10
8
As-weldUIT
S t r e s s r a n g e ,
M P a
Number of cycles to failure
R = 0.1 f = 4 to 9 HzT T
TR
R
R R
T: Weld toe
R: Root
R R
R
R
as-welded
8/13/2019 Paper - Effect of Uit on Fatigue Life in Web-gusset Welded Joints
3.2 Factors affecting the fatigue lives of welded joints
3.2.1 Residual stress
The residual stress distribution on the surface along the cross section at the weld toe of
the AW specimen and that of the UIT specimen were measured using an X-ray diffraction
stress analyzer (Rigaku PSPC-MSF-2) with an X-ray target CrK α. The peak location of theresidual stress was calculated using the half-breadth method and seven Ψ angles were used
to determine the slope of sin2Ψ - 2θ diagram(10)
. The diffraction angle was 156.08°, and the
irradiation area was 4 mm2.
The distributions of the longitudinal residual stress, which was measured using the
X-ray diffraction method, are shown in Figure 7. Although the maximum tensile residual
stress was over 300 MPa at the weld toe in the AW specimen, the residual stress at the weld
toe in the UIT specimen was 200 MPa compressive residual stress. The large amount of
compressive residual stress, which exceeded the amount of tensile residual stress, was
introduced by UIT.
Fig. 7 Residual stress distribution.
3.2.2 Stress concentration factor
The stress concentration factor K t at the weld toes in the AW specimen and that in the
UIT specimen were calculated by performing an elastic stress analysis using the
three-dimensional finite element method (FEM) code MARC. Replicas of the weld zones
were made in silicone rubber. The configurations of the replicas were measured using anoptical microscope with a 10x object lens. In particular, the radii of the weld toes were
accurately measured using a 3-D laser microscope (KEYENCE VK-9510) with a resolution
of 0.01 µm. The radii of the weld toes ρ and the flank angles θ are shown in Table 6. The
radii of weld toes in UIT specimens approximately coincided with the curvature of the pin
tip mounted on the hand tool in the UIT equipment. Figure 8 shows typical FE models using
the measurement results for the weld zones. Given the symmetry of the specimens, one in
eight FE models was meshed with 60,000 elements and 66,000 nodes using
8-node-solid-elements. Young’s modulus was 206 GPa and Poisson’s ratio was 0.3.
The stress contours of σ xx around the weld toes are shown in Figure 9. The stress
concentration is relaxed by the use of UIT. The values for K t , which were calculated from
the FEM results, are shown in Table 7. The K t for the UIT specimens was about 40% lessthat that for the AW specimens.
-200
0
200
400
600
0 20 40 60 80 100
As-weldUIT
R e s i d u a l s t r e s s , M P a
Distance from the edge, mm
00
as-welded
8/13/2019 Paper - Effect of Uit on Fatigue Life in Web-gusset Welded Joints
Microstructures around the weld toes etched with 2.5% Nital were observed using the
laser microscope (KEYENCE VK-9510). Figure 10 shows the microstructures around the
weld toes. Unlike for the AW specimen, the refinement of the grains was observed from the
surface to 300 µm in depth for the UIT specimen. The grain sizes from the surface to 25 µm
in depth at the weld toes were measured using electron backscatter diffraction pattern(EBSD) analysis. The grain boundary was defined as the location at which the change in the
crystal orientation exceeded 15°. Also, the grain size was defined as the diameter of a circle
that was equivalent to the grain in area. Figure 11 shows images of the grain boundary at the
weld toes in the AW specimen and the UIT specimen. The grain sizes in the AW specimen
ranged from 5 to 14 µm. Meanwhile, the grain sizes in the UIT specimen ranged from 0.3 to
0.7 µm, one-tenth or less the grain sizes in the AW specimen.
Fig. 10 Comparison between microstructures around weld toes of AW specimen and UIT
specimen.
Fig. 11 Grain boundary images for the AW specimen and the UIT specimen.
50 µm 50 µm
(a) AW (b) UIT
Gusset
Base metal
Gusset
Base metal
10 µm 10 µm
(a) AW (b) UIT
8/13/2019 Paper - Effect of Uit on Fatigue Life in Web-gusset Welded Joints
The distribution of Vickers hardness (HV) from the surface to 2000 µm in depth at the
weld toe was measured using a nanoindenter (FISCHERSCOPE HM2000). The nanoindenter
measures the load and displacement of the indenter under loading and unloading conditions,
and the Vickers hardness is calculated based on the relationship between the load and the
displacement of the indenter (11)
. The maximum load loaded to the indenter was 2000 mN.
Figure 12 shows the distribution of the Vickers hardness for the AW specimen and the UITspecimen. The Vickers hardness on the surface was increased 1.3 times by the use of UIT. The
depth to which hardness was increased by the use of UIT was from the surface to 300 µm in
depth, which coincided with the depth of the grain refinement observed.
Fig. 12 Distribution of Vickers hardness.
4. Conclusions
In order to examine the effect of UIT on the fatigue lives of web-gusset welded joints,
fatigue tests were carried out on joints that had been treated by UIT and the results were
then compared with the results obtained for as-welded joints. Also, changes in the residual
stress, stress concentration factor, and grain size around the weld toes resulting from the use
of UIT were measured. The following conclusions were obtained.
(1) The fatigue lives of web-gusset welded joints treated by UIT were more than ten times
those of as-welded joints, and the fatigue limit increased from 53 MPa to 111 MPa.
(2) The improvement in the fatigue lives as a result of the use of UIT was caused by an
increase in the fatigue crack initiation life in the stress range of over 175 MPa. In thestress range of 175 MPa or less, the fatigue lives of the joints increased because the
location at which fatigue cracks were initiated changed from the weld toe to the weld
root.
(3) The residual stress on the surface at the weld toe changed from 300 MPa to −200 MPa
after UIT was applied.
(4) The stress concentration factor at the weld toes in the UIT specimens calculated using
FEM decreased by about 40% in comparison with that of AW specimens.
(5) The grain sizes under the weld toe in the UIT specimen was 0.3 to 0.7 µm, one-tenth or
less the grain sizes in the AW specimen.
(6) The Vickers hardness at the weld toe in the UIT specimen increased from the surface to
300 µm in depth, which coincided with the depth of the grain refinement observed.
150
175
200
225
250
275
300
325
350
0 600 1200 1800 2400
As-weldUIT
H a r d n e s s
, H V
Distance from surface, µm
300
Grain refinement
as-welded
8/13/2019 Paper - Effect of Uit on Fatigue Life in Web-gusset Welded Joints